Dielectrics provide electrical insulation to prevent undesired current flow in electrical equipment, including capacitors, transmission lines, circuit substrates, semiconductor devices, mechanical supports, and other applications. Dielectrics are limited to operation at electric fields at or below their dielectric strength, the electric field level at which the dielectric fails to prevent current flow below an acceptable level. The maximum electric field at which a dielectric can reliably be operated is dependent on the applied voltage, the thickness of the dielectric, and the spatial relationship of conductors and dielectrics in the system, among other factors. Since the dielectric strength is limited by the material properties, manufacturing quality, and operating conditions, the thickness of the dielectric often must be increased to enable operation at higher voltage. As the thickness of the dielectric increases to support higher voltage operation, the size of the devices, such as capacitors and transmission lines, increases. In the case of capacitors, increasing the dielectric thickness reduces the capacitance density, defined as the capacitance per unit area, and thus requires a larger capacitor area to obtain an equivalent total capacitance as a capacitor incorporating a thinner dielectric. The additional material required to increase both the thickness and area of the dielectric causes the volume and weight of the dielectric and overall device to be much larger. Since many applications for capacitors and other electrical devices incorporating dielectrics would benefit from smaller and/or lighter components, materials capable of operating at higher electric field are needed to reduce the size and weight of the components incorporating the dielectrics. The electrical energy density, defined as the energy stored in a dielectric per unit volume or per unit mass, is commonly used as a metric to evaluate the size and/or weight of dielectrics based on their material properties. By increasing a dielectric's electrical energy density, components incorporating that dielectric can often be made smaller and lighter.
The electrical energy density, We [J/m3], can be calculated for a medium with a relative permittivity, which is commonly referred to as the dielectric constant, εr, in an electric field, E [V/m], by the following equation. The permittivity of free space, ε0 [F/m], is approximately 8.85×10−12 F/m.
      W    e    =            1      2        ⁢          ɛ      0        ⁢          ɛ      r        ⁢          E      2      
The electrical energy density scales with the square of the electric field and linearly with the dielectric constant. Therefore, increasing the operating electric field, which is limited by the dielectric strength of the medium, and/or the dielectric constant, which is dependent on the dielectric medium and is often a function of the operating temperature, frequency, and electric field, will increase the electrical energy density. While many efforts to increase the electrical energy density have sought to increase both the dielectric constant and the dielectric strength, and thus operating electric field, increasing the dielectric strength of a dielectric medium to increase the operating electric field can provide much more significant gains in electrical energy density due to the squared term.
Electrostatic capacitors store energy in an electric field. The term electrostatic capacitor used here is intended to differentiate from electric double layer capacitors and should be understood to include devices used with an alternating current (AC) or transient signal as well as direct current (DC). While capacitors have conventionally used dielectrics to separate the positive and negative charges of a capacitor, electric double layer capacitors, which are often called supercapacitors or ultracapacitors, do not typically have a dielectric separating the charges. Rather, conducting electrolyte and electrodes are in direct contact, and charges are not transferred across the interface unless the applied voltage is above a threshold voltage of a few volts. Thus, while the capacitance density and energy density of electric double layer capacitors is typically greater than those of electrostatic capacitors, the voltage of electric double layer capacitors is limited. While electric double layer capacitors can be arranged in series to create assemblies with higher operating voltage, either the capacitance of each electric double layer capacitor in the series stack must be approximately equal to evenly distribute the voltage across each element of the assembly or additional voltage control hardware must be used to prevent any individual capacitor from operating at too high of voltage. Thus, electric double layer capacitors are not suitable for providing high energy density in many high voltage applications.
Capacitors capable of operating at higher voltages than electric double layer capacitors (i.e., electrostatic capacitors) typically have a much lower capacitance density and energy density than electric double layer capacitors. There is a need for improvements in the field of electrostatic capacitors to increase energy density and power density while maintaining or improving capacitance density, and thus improvements to the dielectrics in electrostatic capacitors are needed. There are several other important performance factors related to dielectrics for electrostatic capacitors and other applications that can also be improved with new dielectric materials. First, many dielectric applications require both the capability to withstand high and low temperature extremes, temperature cycling, and to have a small variation in dielectric properties with respect to temperature. Second, it is also often critical for there to be minimal variation of the dielectric properties with respect to the applied voltage/operating electric field in the dielectric or other operational or environmental factors. For example, to remain within specification for certain applications, capacitor dielectrics must not change more than a predetermined percentage from their standard value over the range of voltages, frequencies, and temperatures of use. Solid and liquid dielectrics can have large variation due to effects of voltage/electric field, frequency, and temperature due to the dependence of the dielectric constant on these parameters. Conventional dielectrics with high dielectric constants, such as ferroelectrics, often have large variance with environmental and operating variables. In the case of capacitors, stability of the capacitance and voltage rating with respect to temperature and applied voltage would greatly expand the applicability of capacitors with high energy density storage capabilities. Third, many AC and radio frequency (RF) applications require low dielectric losses to operate efficiently at high frequencies. Some capacitors and cables operate with low-loss dielectrics based on foams produced with low dielectric constant polymers (e.g. polyethylene or polypropylene with εr<4), or simply gas or vacuum as the electrical insulator. However, these low-loss dielectrics have poor dielectric strength due to their implementation, limiting their use in devices for high electric field operation for high energy and power density. Fourth, many applications require improved reliability, which typically requires derating of the operating conditions (e.g., operating electric field in the dielectric of a capacitor) from the maximum capability of the dielectric to enable a long operating lifetime. Dielectrics with a higher dielectric strength can be operated with a larger derating factor, thus increasing lifetime and reliability, while operating at the same electric fields of other dielectrics with lower derating factors. Fifth, to achieve operation at very high electric fields, film capacitors, which are commonly made with polymer film dielectrics, have been developed with the ability to be operational after one or more failure events within the dielectric. This capability, commonly referred to as self-clearing or self-healing, enables operation beyond the limitations of the weakest points of the dielectric, where a material defect or contamination would otherwise cause premature failure of the entire device. This self-clearing or self-healing property is enabled both by using very thin electrodes and the appropriate dielectric. The dielectric material is a critical factor in determining the self-clearing or self-healing capability, and some dielectrics have been found to be incompatible with the methods used for self-clearing. Due to the known shortcomings of dielectrics known in the art, a novel approach to dielectric materials is required to satisfy the many requirements on dielectrics over the range of operating conditions.
Materials for High Energy Density Capacitors—Conventional high energy density capacitors are based on solid dielectrics of polymers, ceramics, or composites of polymers and ceramics. In some cases, liquid dielectrics, such as insulating oils, are used in addition to a solid dielectric. Significant efforts have been made in recent decades to mature thin-film polymer manufacturing for high energy density capacitors. Biaxially oriented polypropylene (BOPP) is commercially available in films with thicknesses on the order of microns that can be operated at field strengths on the order of 300 MV/m. These thin-film polymers are routinely used in practice, and the energy density is particularly high when made with self-healing electrodes capable of isolating points of failure in the dielectric through the vaporization of the electrode around the failure point with the surge in current at the fault. These polymer-based thin films are limited in the range of operating temperature due to the properties of the polymer. In particular, the polymer will soften or otherwise degrade at high temperature such that the dielectric strength and energy density are reduced. The polymer properties are also dependent on the glass transition temperature, which can affect low temperature performance. Other acrylic-based polymers with higher operating temperature have been commercialized at thicknesses of less than 1 um. While the small thickness and different polymer chemistry are improvements from other polymer capacitor technologies, the technology has reached its limits with operation demonstrated at near 1,000 MV/m for sub-micron polymer thicknesses.
Other approaches to achieve a high capacitive energy density use high dielectric constant materials. Some polymers, such as poly vinylidene fluoride (PVDF) and other polymers with polar groups, can have dielectric constants greater than 10. However, the dielectric strength of these polar polymers is typically much lower than the previously discussed BOPP and acrylic-based thin-film polymers. The polar groups also cause significant losses under AC signals, which make them unsuitable for many applications. Ceramic materials with dielectric constants ranging up to several thousand have been used in capacitors. The ceramics with the highest dielectric constants are ferroelectrics such as barium titanate and strontium titanate. While these ceramics have many applications, their use in high energy density capacitors is limited due to ferroelectric saturation, which causes a large decrease in the dielectric constant with increased voltage. These perovskite ceramics also exhibit high dielectric losses at RF frequencies and can have significant variation of properties with respect to temperature. Other ceramics are applied in high energy density capacitors. Aluminum electrolytic capacitors operate the aluminum oxide dielectric layer at fields up to 800 MV/m. With a dielectric constant of approximately 10, a field strength of 800 MV/m corresponds to 28.3 J/cm3, though the operating field and energy density is typically lower. Tantalum capacitors, with a dielectric constant of 25, are also used in high energy density applications but typically at lower field strengths than aluminum oxide. While these ceramics are at the state of the art of commercially available capacitors, the opportunities for increases in energy density are limited as the ceramics are already operated near their theoretical breakdown limit (1.38 GV/m for trigonal aluminum oxide and 370 MV/m for cubic tantalum oxide). Another area of active work for high energy density capacitors is polymer-ceramic composites. These composites of ceramic particles in a polymer matrix typically exhibit a higher dielectric constant than the unloaded polymer and a higher dielectric strength than the ceramic of the same thickness of the composite. However, the dielectric strength is often determined by the dielectric strength of the polymer, so the polymer-ceramic composite approach has little or no advantage to polymer multi-layer capacitors in which the polymer layer is on the sub-micron scale as is done in acrylic-based polymer multi-layer capacitors.